Implicit transform selection in video coding

ABSTRACT

An example method includes inferring, for a current transform block of a current video block, a transform type from a plurality of transform types that includes one or more discrete cosine transforms (DCTs) and one or more discrete sine transforms (DSTs), wherein inferring the transform type comprises: determining a size of the current transform block; determining whether the current video block is partitioned using intra-subblock partitioning (ISP); and responsive to determining that the size of the current transform block is less than a threshold and that the current video block is partitioned using ISP, selecting a particular DST of the one or more DSTs as the selected transform type; transforming, using the selected transform type, the current transform block to obtain a block of reconstructed residual data for the video block; and reconstructing, based on the reconstructed residual data for the video block, the video block.

This application claims the benefit of U.S. Provisional PatentApplication 62/817,397, filed on Mar. 12, 2019, the entire content ofwhich is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In one example, a method includes inferring, for a current transformblock of a current video block, a transform type from a plurality oftransform types that includes one or more discrete cosine transforms(DCTs) and one or more discrete sine transforms (DSTs), whereininferring the transform type comprises: determining a size of thecurrent transform block; determining whether the current video block ispartitioned using intra-subblock partitioning (ISP); and responsive todetermining that the size of the current transform block satisfies asize threshold and that the current video block is partitioned usingISP, selecting a particular DST of the one or more DSTs as the selectedtransform type; transforming, using the selected transform type, thecurrent transform block to obtain a block of reconstructed residual datafor the video block; and reconstructing, based on the reconstructedresidual data for the video block, the video block.

In another example, a device includes a memory configured to store videoblocks; and one or more processors implemented in circuitry andconfigured to: infer, for a current transform block of a current videoblock, a transform type from a plurality of transform types thatincludes one or more DCTs and one or more DSTs, wherein, to infer thetransform type, the one or more processors are configured to: determinea size of the current transform block; determine whether the currentvideo block is partitioned using ISP; and select, responsive todetermining that the size of the current transform block satisfies asize threshold and that the current video block is partitioned usingISP, a particular DST of the one or more DSTs as the selected transformtype; transform, using the selected transform type, the currenttransform block to obtain a block of reconstructed residual data for thevideo block; and reconstruct, based on the reconstructed residual datafor the video block, the video block.

In another example, a computer-readable storage medium storesinstructions that, when executed, cause one or more processors of avideo coding device to: infer, for a current transform block of acurrent video block, a transform type from a plurality of transformtypes that includes one or more DCTs and one or more DSTs, wherein theinstructions that cause the one or more processors to infer thetransform type comprise instructions that cause the one or moreprocessors to: determine a size of the current transform block;determine whether the current video block is partitioned using ISP; andselect, responsive to determining that the size of the current transformblock satisfies a size threshold and that the current video block ispartitioned using ISP, a particular DST of the one or more DSTs as theselected transform type; transform, using the selected transform type,the current transform block to obtain a block of reconstructed residualdata for the video block; and reconstruct, based on the reconstructedresidual data for the video block, the video block.

The details of one or more examples of this disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of various aspects of the techniques will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 2C is a conceptual diagram illustrating another example quadtreestructure and corresponding tree unit.

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a block diagram illustrating a system for hybrid videoencoding with adaptive transform selection.

FIG. 6 is a conceptual diagram illustrating separable transformimplementation with horizontal and vertical lines being transformedindependently.

FIG. 7 is a conceptual diagram illustrating an example block for which avideo coder may implicitly derive transforms, in accordance with one ormore techniques of this disclosure.

FIG. 8 is a conceptual diagram illustrating intra prediction directions.

FIG. 9 is a flowchart illustrating an example method for encoding acurrent block.

FIG. 10 is a flowchart illustrating an example method for decoding acurrent block.

FIG. 11 is a flowchart illustrating an example method for inferring atransform type for a transform block of a video block, in accordancewith one or more techniques of this disclosure.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for implicit transformselection in video coding. As discussed in further detail below,following prediction, such as intra-prediction or inter-prediction of ablock, a video encoder may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. The video encoder mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, the video encoder may apply a discrete cosine transform (DCT).In some examples, the video encoder may utilize different types oftransforms. For instance, the video encoder may use various types ofDCT.

A video decoder may apply an inverse transform when decoding the videodata. Where the video coder may utilize different types of transforms,it may be necessary for the video decoder to determine which transformwas used by the video encoder. In some examples, the video encoder mayexplicitly signal (e.g., encode a syntax element with a value thatindicates) which type of transform was used when transforming theresidual data. However, in some examples, it may not be desirable toexplicitly signal the type of transform used (e.g., due to signalingoverhead).

In accordance with one or more techniques of this disclosure, the videodecoder may implicitly determine which type of transform was used whentransforming the residual data. For instance, the video decoder mayapply a set of rules to determine which type of transform was used whentransforming the residual data based on side information available atthe video decoder (e.g., either explicitly signaled or implicitlyderived from signaled information). The video encoder may apply the samerules when determining which type of transform to use. As such, thevideo encoder and video decoder may both determine which type oftransform to use without explicit signaling of the transform type.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,uncoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch smartphones, televisions, cameras, display devices, digital mediaplayers, video gaming consoles, video streaming device, or the like. Insome cases, source device 102 and destination device 116 may be equippedfor wireless communication, and thus may be referred to as wirelesscommunication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for implicittransform selection. Thus, source device 102 represents an example of avideo encoding device, while destination device 116 represents anexample of a video decoding device. In other examples, a source deviceand a destination device may include other components or arrangements.For example, source device 102 may receive video data from an externalvideo source, such as an external camera. Likewise, destination device116 may interface with an external display device, rather than includingan integrated display device.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques forimplicit transform selection. Source device 102 and destination device116 are merely examples of such coding devices in which source device102 generates coded video data for transmission to destination device116. This disclosure refers to a “coding” device as a device thatperforms coding (encoding and/or decoding) of data. Thus, video encoder200 and video decoder 300 represent examples of coding devices, inparticular, a video encoder and a video decoder, respectively. In someexamples, devices 102, 116 may operate in a substantially symmetricalmanner such that each of devices 102, 116 include video encoding anddecoding components. Hence, system 100 may support one-way or two-wayvideo transmission between video devices 102, 116, e.g., for videostreaming, video playback, video broadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, uncoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some example, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although shown separately from video encoder 200 and videodecoder 300 in this example, it should be understood that video encoder200 and video decoder 300 may also include internal memories forfunctionally similar or equivalent purposes. Furthermore, memories 106,120 may store encoded video data, e.g., output from video encoder 200and input to video decoder 300. In some examples, portions of memories106, 120 may be allocated as one or more video buffers, e.g., to storeraw, decoded, and/or encoded video data.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may modulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 116. Similarly, destination device 116may access encoded data from storage device 116 via input interface 122.Storage device 116 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., DSL, cable modem, etc.), or acombination of both that is suitable for accessing encoded video datastored on file server 114. File server 114 and input interface 122 maybe configured to operate according to a streaming transmission protocol,a download transmission protocol, or a combination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receiver, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., storage device 112,file server 114, or the like). The encoded video bitstreamcomputer-readable medium 110 may include signaling information definedby video encoder 200, which is also used by video decoder 300, such assyntax elements having values that describe characteristics and/orprocessing of video blocks or other coded units (e.g., slices, pictures,groups of pictures, sequences, or the like). Display device 118 displaysdecoded pictures of the decoded video data to a user. Display device 118may represent any of a variety of display devices such as a cathode raytube (CRT), a liquid crystal display (LCD), a plasma display, an organiclight emitting diode (OLED) display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 4),” Joint Video Experts Team (JVET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 13^(th) Meeting:Marrakech, MA, 9-18 January 2019, JVET-M1001-v6 (hereinafter “VVC Draft4”). The techniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to JEM or VVC. According to JEM or VVC,a video coder (such as video encoder 200) partitions a picture into aplurality of coding tree units (CTUs). Video encoder 200 may partition aCTU according to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) partitions. A triple tree partition is apartition where a block is split into three sub-blocks. In someexamples, a triple tree partition divides a block into three sub-blockswithout dividing the original block through the center. The partitioningtypes in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of JEM and VVC also provide an affine motion compensationmode, which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofJEM and VVC provide sixty-seven intra-prediction modes, includingvarious directional modes, as well as planar mode and DC mode. Ingeneral, video encoder 200 selects an intra-prediction mode thatdescribes neighboring samples to a current block (e.g., a block of a CU)from which to predict samples of the current block. Such samples maygenerally be above, above and to the left, or to the left of the currentblock in the same picture as the current block, assuming video encoder200 codes CTUs and CUs in raster scan order (left to right, top tobottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As discussed above, a video encoder, such as video encoder 200, mayapply various types of transforms to transform the residual data. Thefollowing is an overview of discrete sine and cosine transforms (DCTsand DSTs). Also, the transform scheme used in HEVC standard is brieflydiscussed.

Discrete sine and cosine transforms.

Transform indicates the process of deriving an alternativerepresentation of the input signal. Given an N-point vector x=[x₀, x₁, .. . , x_(N-1)]^(T) and a set of given vectors {Φ₀, Φ₁, . . . , Φ_(M-1)},x can be approximated or exactly represented using a linear combinationof Φ₀, Φ₁, . . . , Φ_(M-1), which can be formulated as follows,

$\overset{\hat{}}{x} = {\sum\limits_{i = 0}^{M - 1}{f_{i} \cdot \Phi_{i}}}$

where {circumflex over (x)} can be an approximation or equivalent of x,vector f=[f₁, f₂, . . . , f_(M-1)] is called the transform coefficientvector and {Φ₀, Φ₁, . . . , Φ_(M-1)} are the transform basis vectors.

In the scenario of video coding, transform coefficients are roughlynon-correlated and sparse, i.e., the energy of the input vector x iscompacted only on a few transform coefficients, and the remainingmajority transform coefficients are typically close to 0.

Given the specific input data, the optimal transform in terms of energycompaction is the so-called Karhunen-Loeve transform (KLT), which usesthe eigen vectors of the covariance matrix of the input data as thetransform basis vectors. Therefore, KLT is actually a data-dependenttransform and does not have a general mathematical formulation. However,under certain assumptions, e.g., the input data forms a first-orderstationary Markov processes, it has been proved in the literature thatthe corresponding KLT is actually a member of the sinusoidal family ofunitary transforms. The sinusoidal family of unitary transformsindicates transforms using transform basis vectors formulated asfollows:

Φ_(m)(k)=A·e ^(ike) +B ^(−ike)

where e is the base of the natural logarithm approximately equal to2.71828, A, B, and θ are complex in general, and depend on the value ofm.

Example transforms include the discrete Fourier, cosine, sine, and theKLT (for first-order stationary Markov processes) are members of thissinusoidal family of unitary transforms. According to S. A. Martucci,“Symmetric convolution and the discrete sine and cosine transforms,”IEEE Trans. Sig. Processing SP-42, 1038-1051 (1994), the complete set ofdiscrete cosine transform (DCT) and discrete sine transform (DST)families includes totally 16 transforms based on different types, i.e.,different values of A, B, and θ, and a complete definition of thedifferent types of DCT and DST are given below,

Assume the input N-point vector is denoted as x=[x₀, x₁, . . . ,x_(N-1)]^(T), and it is transformed to another N-point transformcoefficient vector denoted as y=[y₀, y₁, . . . , y_(N-1)]^(T) bymultiplying a matrix, the process of which can be further illustratedaccording to one of the following transform formulation, wherein kranges from 0 through N-1, inclusive:

DCT Type-I (DCT-1):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N - 1}}{{\cos \left( \frac{\pi \cdot n \cdot k}{N - 1} \right)} \cdot w_{0} \cdot w_{1} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ {\begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = {{0\mspace{14mu} {or}\mspace{14mu} n} = {N - 1}}} \\{1,} & {otherwise}\end{matrix},{w_{1} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = {{0\mspace{14mu} {or}\mspace{14mu} k} = {N - 1}}} \\{1,} & {otherwise}\end{matrix} \right.}} \right.}$

DCT Type-II (DCT-2):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\cos \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot k}{N - 1} \right)} \cdot w_{0} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = 0} \\{1,} & {otherwise}\end{matrix} \right.}$

DCT Type-III (DCT-3):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\cos \left( \frac{{\pi \cdot n} + \left( {k + 0.5} \right)}{N - 1} \right)} \cdot w_{0} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = 0} \\{1,} & {otherwise}\end{matrix} \right.}$

DCT Type-IV (DCT-4):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\cos \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 0.5} \right)}{N} \right)} \cdot x_{n}}}}},$

DCT Type-V (DCT-5):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N - 0.5}}{{\cos \left( \frac{\pi \cdot n \cdot k}{N - 0.5} \right)} \cdot w_{0} \cdot w_{1} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ {\begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = 0} \\{1,} & {otherwise}\end{matrix},{w_{1} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = 0} \\{1,} & {otherwise}\end{matrix} \right.}} \right.}$

DCT Type-VI (DCT-6):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N - 0.5}}{{\cos \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot k}{N - 0.5} \right)} \cdot w_{0} \cdot w_{1} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ {\begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = {N - 1}} \\{1,} & {otherwise}\end{matrix},{w_{1} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = 0} \\{1,} & {otherwise}\end{matrix} \right.}} \right.}$

DCT Type-VII (DCT-7):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N - 0.5}}{{\cos \left( \frac{\pi \cdot n \cdot \left( {k + 0.5} \right)}{N - 0.5} \right)} \cdot w_{0} \cdot w_{1} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ {\begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = 0} \\{1,} & {otherwise}\end{matrix},{w_{1} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = {N - 1}} \\{1,} & {otherwise}\end{matrix} \right.}} \right.}$

DCT Type-VIII (DCT-8):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N + 0.5}}{{\cos \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 0.5} \right)}{N + 0.5} \right)} \cdot x_{n}}}}},$

DST Type-I (DST-1):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N + 1}}{{\sin \left( \frac{\pi \cdot \left( {n + 1} \right) \cdot \left( {k + 1} \right)}{N + 1} \right)} \cdot x_{n}}}}},$

DST Type-II (DST-2):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\sin \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 1} \right)}{N} \right)} \cdot w_{0} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = {N - 1}} \\{1,} & {otherwise}\end{matrix} \right.}$

DST Type-III (DST-3):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\sin \left( \frac{\pi \cdot \left( {n + 1} \right) \cdot \left( {k + 0.5} \right)}{N} \right)} \cdot w_{0} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = {N - 1}} \\{1,} & {otherwise}\end{matrix} \right.}$

DST Type-IV (DST-4):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N}}{{\sin \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 0.5} \right)}{N} \right)} \cdot x_{n}}}}},$

DST Type-V (DST-5):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N + 0.5}}{{\sin \left( \frac{\pi \cdot \left( {n + 1} \right) \cdot \left( {k + 1} \right)}{N + 0.5} \right)} \cdot x_{n}}}}},$

DST Type-VI (DST-6):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N + 0.5}}{{\sin \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 1} \right)}{N + 0.5} \right)} \cdot x_{n}}}}},$

DST Type-VII (DST-7):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N + 0.5}}{{\sin \left( \frac{\pi \cdot \left( {n + 1} \right) \cdot \left( {k + 0.5} \right)}{N + 0.5} \right)} \cdot x_{n}}}}},$

DST Type-VIII (DST-8):

${y_{k} = {\sum_{n = 0}^{N - 1}{\sqrt{\frac{2}{N - 0.5}}{{\cos \left( \frac{\pi \cdot \left( {n + 0.5} \right) \cdot \left( {k + 0.5} \right)}{N - 0.5} \right)} \cdot w_{0} \cdot w_{1} \cdot x_{n}}}}},{{{where}\mspace{14mu} w_{0}} = \left\{ {\begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} n} = {N - 1}} \\{1,} & {otherwise}\end{matrix},{w_{1} = \left\{ \begin{matrix}{\frac{1}{\sqrt{2}},} & {{{if}\mspace{14mu} k} = {N - 1}} \\{1,} & {otherwise}\end{matrix} \right.}} \right.}$

The transform type is specified by the mathematical formulation of thetransform basis function, e.g., 4-point DST-VII and 8-point DST-VII havethe same transform type, regardless the value of N.

Without loss of generality, all the above transform types can berepresented using the below generalized formulation:

y _(m)=Σ_(n=0) ^(N−1) T _(m,n) ·x _(n),

where T is the transform matrix specified by the definition of onecertain transform, e.g., DCT Type-I˜DCT Type-VIII, or DST Type-I˜DSTType-VIII, and the row vectors of T, e.g., [T_(i,0), T_(i,1), . . . ,T_(i,N-1)] are the i^(th) transform basis vectors. A transform appliedon the N-point input vector is called an N-point transform.

It is also noted that, the above transform formulations, which areapplied on the 1-D input data x, can be represented in matrixmultiplication form as below

y=T·x

where T indicates the transform matrix, x indicates the input datavector, and y indicates the output transform coefficients vector.

Transform for 2-Dimensional (2-D) input data.

The transforms as introduced in the previous section are applied on 1-Dinput data, and transforms can be also extended for 2-D input datasources. Supposing X is an input M×N data array. The typical methods ofapplying transform on 2-D input data include the separable andnon-separable 2-D transforms.

A separable 2-D transform applies 1-D transforms for the horizontal andvertical vectors of X sequentially, formulated as below:

Y=C·X·R ^(T)

where C and R denotes the given M×M and N×N transform matrices,respectively. From the formulation, it can be seen that C applies 1-Dtransforms for the column vectors of X, while R applies 1-D transformsfor the row vectors of X In the later part of this document, forsimplicity denote C and R as left (vertical) and right (horizontal)transforms and they both form a transform pair. There are cases when Cis equal to R and is an orthogonal matrix. In such a case, the separable2-D transform is determined by just one transform matrix.

A non-separable 2-D transform first reorganized all the elements of Xinto a single vector, namely X′, by doing the following mathematicalmapping as an example:

X′_((i·N+j))=X_(i,j)

Then a 1-D transform T′ is applied for X′ as below:

Y=T′·X

where T′ is an (M*N)×(M*N) transform matrix.

In video coding, separable 2-D transforms may be applied as it mayutilize much less operation (addition, multiplication) counts ascompared to 1-D transform.

In conventional video codecs, such as H.264/AVC, an integerapproximation of the 4-point and 8-point Discrete Cosine Transform (DCT)Type-II is always applied for both Intra and Inter prediction residual.To better accommodate the various statistics of residual samples, moreflexible types of transforms other than DCT Type-II are utilized innewer video codecs. For example, in HEVC, an integer approximation ofthe 4-point Type-VII Discrete Sine Transform (DST) is utilized for Intraprediction residual, which is both theoretically proved andexperimentally validated that DST Type-VII is more efficient than DCTType-II for residuals vectors generated along the Intra predictiondirections, e.g., DST Type-VII is more efficient than DCT Type-II forrow residual vectors generated by the horizontal Intra predictiondirection. In HEVC, an integer approximation of 4-point DST Type-VII isapplied only for 4×4 luma Intra prediction residual blocks. The 4-pointDST-VII used in HEVC is shown below,

4×4 DST-VII:

{29, 55, 74, 84}{74, 74, 0,−74}{84, −29,−74, 55}{55,−84, 74,−29}

In HEVC, for residual blocks that are not 4×4 luma Intra predictionresidual blocks, integer approximations of the 4-point, 8-point,16-point and 32-point DCT Type-II are also applied, as shown below:

4-point DCT-II:

{64, 64, 64, 64}{83, 36,−36,−83}{64,−64,−64, 64}{36,−83, 83,−36}

8-point DCT-II:

{64, 64, 64, 64, 64, 64, 64, 64}{89, 75, 50, 18,−18,−50,−75,−89}{83, 36,−36,−83,−83,−36, 36, 83}{75,−18,−89,−50, 50, 89, 18,−75}{64,−64,−64, 64, 64,−64,−64, 64}{50,−89, 18, 75,−75,−18, 89,−50}{36,−83, 83,−36, −36, 83,−83, 36}{18,−50, 75,−89, 89,−75, 50,−18}

16-point DCT-II:

{64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64}{90, 87, 80, 70, 57, 43, 25, 9,−9,−25,−43,−57,−70,−80,−87,−90}{89, 75, 50, 18,−18,−50,−75,−89,−89,−75,−50,−18, 18, 50, 75, 89}{87, 57, 9,−43,−80,−90,−70,−25, 25, 70, 90, 80, 43,−9, −57,−87}{83, 36,−36,−83,−83,−36, 36, 83, 83, 36,−36,−83,−83,−36, 36, 83}{80, 9,−70,−87,−25, 57, 90, 43,−43, −90,−57, 25, 87, 70,−9,−80}{75,−18,−89,−50, 50, 89, 18,−75,−75, 18, 89, 50,−50,−89,−18, 75}{70,−43,−87, 9, 90, 25,−80,−57, 57, 80,−25,−90,−9, 87, 43,−70}{64,−64,−64, 64, 64,−64,−64, 64, 64,−64,−64, 64, 64,−64,−64, 64}{57,−80,−25, 90,−9,−87, 43, 70, −70,−43, 87, 9,−90, 25, 80,−57}{50 −89, 18, 75,−75,−18, 89,−50,−50, 89,−18,−75, 75, 18,−89, 50}{43,−90, 57, 25,−87, 70, 9,−80, 80,−9,−70, 87,−25,−57, 90,−43}{36,−83, 83,−36,−36, 83,−83, 36, 36,−83, 83,−36,−36, 83,−83, 36}{25 −70, 90,−80, 43, 9,−57, 87,−87, 57,−9,−43, 80,−90, 70,−25}{18,−50, 75,−89, 89,−75, 50,−18,−18, 50,−75, 89,−89, 75,−50, 18}{9,−25, 43,−57, 70,−80, 87,−90, 90,−87, 80,−70, 57,−43, 25,−9}

32-point DCT-II:

{64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64}{90,90,88,85,82,78,73,67,61,54,46,38,31,22,13,4,−4,−13,−22,−31,−38,−46,−54,−61,−67,−73,−78,−82,−85,−88,−90,−90}{90,87,80,70,57,43,25,9,−9,−25,−43,−57,−70,−80,−87,−90,−90,−87,−80,−70,−57,−43,−25,−9,9,25,43,57,70,80,87,90}{90,82,67,46,22,−4,−31,−54,−73,−85,−90,−88,−78,−61,−38,−13,13,38,61,78,88,90,85,73,54,31,4,−22,−46,−67,−82,−90}{88,67,31,−13,−54,−82,−90,−78,−46,−4,38,73,90,85,61,22,−22,−61,−85,−90,−73,−38,4,46,78,90,82,54,13,−31,−67,−88}{87,57,9,−43,−80,−90,−70,−25,25,70,90,80,43,−9,−57,−87,−87,−57,−9,43,80,90,70,25,−25,−70,−90,−80,−43,9,57,87}{85,46,−13,−67,−90,−73,−22,38,82,88,54,−4,−61,−90,−78,−31,31,78,90,61,4,−54,−88,−82,−38,22,73,90,67,13,−46,−85}{83,36,−36,−83,−83,−36,36,83,83,36,−36,−83,−83,−36,36,83,83,36,−36,−83,−83,−36,36,83,83,36,−36,−83,−83,−36,36,83}{82,22,−54,−90,−61,13,78,85,31,−46,−90,−67,4,73,88,38,−38,−88,−73,−4,67,90,46,−31,−85,−78,−13,61,90,54,−22,−82}{80,9,−70,−87,−25,57,90,43,−43,−90,−57,25,87,70,−9,−80,−80,−9,70,87,25,−57,−90,−43,43,90,57,−25,−87,−70,9,80}{78,−4,−82,−73,13,85,67,−22,−88,−61,31,90,54,−38,−90,−46,46,90,38,−54,−90,−31,61,88,22,−67,−85,−13,73,82,4,−78}{73,−31,−90,−22,78,67,−38,−90,−13,82,61,−46,−88,−4,85,54,−54,−85,4,88,46,−61,−82,13,90,38,−67,−78,22,90,31,−73}{70,−43,−87,9,90,25,−80,−57,57,80,−25,−90,−9,87,43,−70,−70,43,87,−9,−90,−25,80,57,−57,−80,25,90,9,−87,−43,70}{67,−54,−78,38,85,−22,−90,4,90,13,−88,−31,82,46,−73,−61,61,73,−46,−82,31,88,−13,−90,−4,90,22,−85,−38,78,54,−67}{64,−64,−64,64,64,−64,−64,64,64,−64,−64,64,64,−64,−64,64,64,−64,−64,64,64,−64,−64,64,64,−64,−64,64,64,−64,−64,64}{61,−73,−46,82,31,−88,−13,90,−4,−90,22,85,−38,−78,54,67,−67,−54,78,38,−85,−22,90,4,−90,13,88,−31,−82,46,73,−61}{57,−80,−25,90,−9,−87,43,70,−70,−43,87,9,−90,25,80,−57,−57,80,25,−90,9,87,−43,−70,70,43,−87,−9,90,−25,−80,57}{54,−85,−4,88,−46,−61,82,13,−90,38,67,−78,−22,90,−31,−73,73,31,−90,22,78,−67,−38,90,−13,−82,61,46,−88,4,85,−54}{46,−90,38,54,−90,31,61,−88,22,67,−85,13,73,−82,4,78,−78,−4,82,−73,−13,85,−67,−22,88,−61,−31,90,−54,−38,90,−46}{43,−90,57,25,−87,70,9,−80,80,−9,−70,87,−25,−57,90,−43,−43,90,−57,−25,87,−70,−9,80,−80,9,70,−87,25,57,−90,43}{25,−70,90,−80,43,9,−57,87,−87,57,−9,−43,80,−90,70,−25,−25,70,−90,80,−43,−9,57,−87,87,−57,9,43,−80,90,−70,25}{22,−61,85,−90,73,−38,−4,46,−78,90,−82,54,−13,−31,67,−88,88,−67,31,13,−54,82,−90,78,−46,4,38,−73,90,−85,61,−22}{13,−38,61,−78,88,−90,85,−73,54,−31,4,22,−46,67,−82,90,−90,82,−67,46,−22,−4,31,−54,73,−85,90,−88,78,−61,38,−13}{9,−25,43,−57,70,−80,87,−90,90,−87,80,−70,57,−43,25,−9,−9,25,−43,57,−70,80,−87,90,−90,87,−80,70,−57,43,−25,9}{4,−13,22,−31,38,−46,54,−61,67,−73,78,−82,85,−88,90,−90,90,−90,88,-85,82,−78,73,−67,61,−54,46,−38,31,−22,13,−4}

Transform scheme based on residual quadtree in HEVC.

To adapt the various characteristics of the residual blocks, a transformcoding structure using the residual quadtree (RQT) is applied in HEVC,which is briefly described inhttp://www.hhi.fraunhofer.de/fields-of-competence/image-processing/research-groups/image-video-coding/hevc-high-efficiency-video-coding/transform-coding-using-the-residual-quadtree-rqt.html.In RQT, each picture is divided into coding tree units (CTU), which arecoded in raster scan order for a specific tile or slice. A CTU is asquare block and represents the root of a quadtree, i.e., the codingtree. The CTU size may range from 8×8 to 64×64 luma samples, buttypically 64×64 is used. Each CTU can be further split into smallersquare blocks called coding units (CUs). After the CTU is splitrecursively into CUs, each CU is further divided into prediction units(PU) and transform units (TU).

The partitioning of a CU into TUs may be carried out recursively basedon a quadtree approach, therefore the residual signal of each CU iscoded by a tree structure namely, the residual quadtree (RQT). The RQTallows TU sizes from 4×4 up to 32×32 luma samples. FIG. 2C shows anexample where a CU includes 10 TUs, labeled with the letters a to j, andthe corresponding block partitioning. Each node of the RQT is actually atransform unit (TU). The individual TUs may be processed in depth-firsttree traversal order, which is illustrated in the figure as alphabeticalorder, which follows a recursive Z-scan with depth-first traversal.

The quadtree approach enables the adaptation of the transform to thevarying space-frequency characteristics of the residual signal.Typically, larger transform block sizes, which have larger spatialsupport, provide better frequency resolution. However, smaller transformblock sizes, which have smaller spatial support, may provide betterspatial resolution. The trade-off between the two, spatial and frequencyresolutions, may be chosen by the encoder mode decision, for examplebased on rate-distortion optimization technique. The video coder mayperform a rate-distortion optimization technique to calculate a weightedsum of coding bits and reconstruction distortion, i.e., therate-distortion cost, for each coding mode (e.g., a specific RQTsplitting structure), and select the coding mode with leastrate-distortion cost as the best mode.

Three parameters may be defined in the RQT: the maximum depth of thetree, the minimum allowed transform size and the maximum allowedtransform size. The minimum and maximum transform sizes can vary withinthe range from 4×4 to 32×32 samples, which correspond to the supportedblock transforms mentioned in the previous paragraph. The maximumallowed depth of the RQT restricts the number of TUs. A maximum depthequal to zero means that a CB cannot be split any further if eachincluded TB reaches the maximum allowed transform size, e.g., 32×32.

All these parameters interact and influence the RQT structure. Considera case, in which the root CB size is 64×64, the maximum depth is equalto zero and the maximum transform size is equal to 32×32. In this case,the CB has to be partitioned at least once, since otherwise it wouldlead to a 64×64 TB, which is not allowed. The RQT parameters, i.e.maximum RQT depth, minimum and maximum transform size, are transmittedin the bitstream at the sequence parameter set level. Regarding the RQTdepth, different values can be specified and signaled for intra andinter coded CUs.

The quadtree transform is applied for both Intra and Inter residualblocks. Typically the DCT-II transform of the same size of the currentresidual quadtree partition is applied for a residual block. However, ifthe current residual quadtree block is 4×4 and is generated by Intraprediction, the above 4×4 DST-VII transform is applied.

In HEVC, larger size transforms, e.g., 64×64 transform are not adoptedmainly due to its limited benefit considering and relatively highcomplexity for relatively smaller resolution videos.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of thecoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) coefficients at the front of the vector and to place lowerenergy (and therefore higher frequency) transform coefficients at theback of the vector. In some examples, video encoder 200 may utilize apredefined scan order to scan the quantized transform coefficients toproduce a serialized vector, and then entropy encode the quantizedtransform coefficients of the vector. In other examples, video encoder200 may perform an adaptive scan. After scanning the quantized transformcoefficients to form the one-dimensional vector, video encoder 200 mayentropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information of apicture into CTUs, and partitioning of each CTU according to acorresponding partition structure, such as a QTBT structure, to defineCUs of the CTU. The syntax elements may further define prediction andresidual information for blocks (e.g., CUs) of video data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

In accordance with the techniques of this disclosure, a video coder(i.e., video encoder 200 and/or video decoder 300) may derive, for acurrent coefficient block of a video block, a transform type from aplurality of transform types. The video coder may transform, using theselected transform type, the current transform block (e.g., coefficientblock) to obtain a block of reconstructed residual data for the videoblock; and reconstruct, based on the reconstructed residual data for thevideo block, the video block.

The video coder may infer the transform type based on factors other thanan express signaling of the transform type. As such, the video coder mayomit coding of a syntax element that expressly identifies the transformtype for a current block. Some examples of factors from-which the videocoder may infer the transform type include, a size of the current block(e.g., a height and/or a width of the current block), whether thecurrent block is partitioned using intra-subblock partitioning (ISP),and an intra mode of the current block. The video coder may infer thetransform type based on any combination of factors. For instance, thevideo coder may infer the transform type for a current transform blockof a current video block based on a size of the current transform blockand whether the current video block is partitioned using ISP. In atleast some of such examples, the video coder may infer the transformtype for the current transform block regardless of an intra predictionmode used to predict the current video block.

The video coder may select the transform type from a plurality oftransform types that includes one or more discrete cosine transforms(DCTs) and one or more discrete sine transforms (DSTs). As discussed infurther detail below, the one or more DCTs may include one or more of aDCT-1, a DCT-2, a DCT-3, a DCT-4, a DCT-5, a DCT-6, a DCT-7, and aDCT-8, and/or the one or more DSTs may include one or more of a DST-1, aDST-2, a DST-3, a DST-4, a DST-5, a DST-6, a DST-7, and a DST-8.

As discussed above, the video coder may infer the transform type for acurrent transform block based on a size of the current transform block.For instance, the video coder may select a first transform type for thecurrent transform block responsive to determining that the size of thecurrent transform block satisfies a size threshold and select a secondtransform type for the current transform block responsive to determiningthat the size of the current transform block does not satisfy the sizethreshold. In some examples, the video coder may determine whether thesize of the current transform block satisfies the size threshold bycomparing the size of the current transform block to a single thresholdvalue. In other examples, the video coder may determine whether the sizeof the current transform block satisfies the size threshold bydetermining whether the size of the current transform block is greaterthan a lower bound (e.g., 2, 4, 6) and less than an upper bound (e.g.,8, 16, 32). If the size of the current transform block is greater thanthe lower bound and less than the upper bound, the video coder maydetermine that the size of the current transform block satisfies thesize threshold. Similarly, if the size of the current transform block isless than the lower bound or greater than the upper bound, the videocoder may determine that the size of the current transform block doesnot satisfy the size threshold.

Where the current video block is coding unit (CU), the CU may bepartitioned into a plurality of sub-partitions using ISP. Each of thesub-partitions may have an associated transform block. As such, wherethe CU is partitioned using ISP, a plurality of transform blocks may beassociated with the CU. For instance, a 16×16 CU may be verticallypartitioned into four partitions of size 4×16, each of which isassociated with a transform block of size 4×16.

As discussed above, the video coder may infer the transform type for acurrent transform block of a current video block based on whether thecurrent video block is partitioned using ISP and based on a size of thecurrent transform block. As one example, responsive to determining thatthe size of the current transform block satisfies a size threshold andthat the current video block is partitioned using ISP, the video codermay select a particular DST of the one or more DSTs (e.g., DST-7) as thetransform type for the current transform block. As another example,responsive to determining that the size of the current transform blockdoes not satisfy the size threshold and that the current video block ispartitioned using ISP, the video coder may select a particular DCT ofthe one or more DCTs (e.g., DCT-2) as the transform type for the currenttransform block. In either of the aforementioned examples, the videocoder may select the transform type comprises selecting the transformtype regardless of an intra prediction mode used to predict the currentvideo block (e.g., regardless of the angular, DC, or planar mode used tointra predict the current video).

In some examples, the video coder may always perform the transform typeinference. In other examples, the video coder may perform the transformtype inference under certain conditions. For instance, the video codermay infer the transform type for the current transform block responsiveto determining that multiple transform selection (MTS) is enabled forthe current video block. The video coder may, in some examples,determine whether MTS is enabled for the current video block based onthe values of one or more syntax elements (e.g.,sps_explicit_mts_intra_enabled_flag).

The video coder may, in some examples, infer a transform type forperforming horizontal transformations (i.e., a transform type forhorizontal use) and infer a transform type for performing verticaltransformations (i.e., a transform type for vertical use). The videocoder may infer the transform types for horizontal and vertical useusing a common algorithm. For instance, the video coder may infer thetransform type for horizontal use based on whether a width of a currenttransform block satisfies a width size threshold and whether a currentvideo block that includes the current transform block is partitionedusing ISP, and infer the transform type for vertical use based onwhether a height of the current transform block satisfies a height sizethreshold and whether the current video block that includes the currenttransform block is partitioned using ISP. In some examples, the videocoder may use the same size threshold for both horizontal and verticaltransform type inferences. For instance, where the size thresholdsinclude an upper bound and a lower bound, the upper and lower bounds ofthe width size threshold may be equal to the upper and lower bounds ofthe height size threshold. As one specific example, the lower bound ofboth width and height thresholds may be 4 and the upper bound of bothwidth and height thresholds may be 16.

In some examples, to derive (i.e., infer) the transform type for thecurrent coefficient block, the video coder may select the DST-7transform to transform any row or column with less than or equal to athreshold (e.g., 8, 16, 32) number of samples (e.g., luma samples) andselect the DCT-2 transform to transform any row or column with greaterthan the threshold number of samples.

Relative to VVC Draft 4 (e.g., JVET-M1001), an example of the proposedchange can be achieved by replacing the Table 8-15 with the following:

trTypeHor=(nTbW>=2 && nTbW<=16)?1:0

trTypeVer=(nTbH>=2 && nTbH<=16)?1:0

where “0” and “1” denote DCT-2 and DST-7 respectively.

Blocks partitioned using ISP may be prohibited from having rows/columnswith only two samples. As such, this disclosure proposes a 2-pointDST-7. The entries of the 2-point DST-7 matrix may be as follows (whichonly introduces 4-bytes of additional memory):

{48  77} {77   − 48}

Alternatively, an example of the proposed change can be achieved bymodifying VVC Draft 4 as follows:

trTypeHor=(nTbW>=4 && nTbW<=16 && nTbW<=nTbH)?1:0   (8-1029)

trTypeVer=(nTbH>=4 && nTbH<=16 && nTbH<=nTbW)?1:0   (8-1030)

where “0” and “1” denote DCT-2 and DST-7 respectively and the changes(i.e., deleted portions) are in underline and italics.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values syntax elements and/or other data used to decodeencoded video data. That is, video encoder 200 may signal values forsyntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagram illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, since quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), they can be furtherpartitioned by respective binary trees. The binary tree splitting of onenode can be iterated until the nodes resulting from the split reach theminimum allowed binary tree leaf node size (MinBTSize) or the maximumallowed binary tree depth (MaxBTDepth). The example of QTBT structure130 represents such nodes as having dashed lines for branches. Thebinary tree leaf node is referred to as a coding unit (CU), which isused for prediction (e.g., intra-picture or inter-picture prediction)and transform, without any further partitioning. As discussed above, CUsmay also be referred to as “video blocks” or “blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, it will not be further split by thebinary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in thisexample). Otherwise, the leaf quadtree node will be further partitionedby the binary tree. Therefore, the quadtree leaf node is also the rootnode for the binary tree and has the binary tree depth as 0. When thebinary tree depth reaches MaxBTDepth (4, in this example), no furthersplitting is permitted. When the binary tree node has width equal toMinBTSize (4, in this example), it implies no further horizontalsplitting is permitted. Similarly, a binary tree node having a heightequal to MinBTSize implies no further vertical splitting is permittedfor that binary tree node. As noted above, leaf nodes of the binary treeare referred to as CUs, and are further processed according toprediction and transform without further partitioning.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 video coding standard in development. However,the techniques of this disclosure are not limited to these video codingstandards, and are applicable generally to video encoding and decoding.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video encoder 200 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that canprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, the one or more of the units maybe distinct circuit blocks (fixed-function or programmable), and in someexamples, the one or more units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theobject code of the software that video encoder 200 receives andexecutes, or another memory within video encoder 200 (not shown) maystore such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 210 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,uncoded version of the current block from video data memory 230 and theprediction block from mode selection unit 202. Residual generation unit204 calculates sample-by-sample differences between the current blockand the prediction block. The resulting sample-by-sample differencesdefine a residual block for the current block. In some examples,residual generation unit 204 may also determine differences betweensample values in the residual block to generate a residual block usingresidual differential pulse code modulation (RDPCM). In some examples,residual generation unit 204 may be formed using one or more subtractorcircuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit does not further partition a CUinto PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 120 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as fewexamples, mode selection unit 202, via respective units associated withthe coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block. As discussed herein, transform processing unit 206may selectively apply different transforms to different coefficientblocks (i.e., blocks of transform coefficients).

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the coefficient blocks associated withthe current block by adjusting the QP value associated with the CU.Quantization may introduce loss of information, and thus, quantizedtransform coefficients may have lower precision than the originaltransform coefficients produced by transform processing unit 206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 224 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 224 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding blocks andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data including a memory configured to store video data, and one ormore processing units implemented in circuitry and configured to derive,for a current coefficient block of a video block, a transform type froma plurality of transform types. The video coder may transform, using theselected transform type, the current coefficient block to obtain a blockof reconstructed residual data for the video block; and reconstruct,based on the reconstructed residual data for the video block, the videoblock.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 is describedaccording to the techniques of JEM, VVC, and HEVC. However, thetechniques of this disclosure may be performed by video coding devicesthat are configured to other video coding standards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. Moreover, video decoder 300 mayinclude additional or alternative processors or processing circuitry toperform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeaddition units to perform prediction in accordance with other predictionmodes. As examples, prediction processing unit 304 may include a paletteunit, an intra-block copy unit (which may form part of motioncompensation unit 318), an affine unit, a linear model (LM) unit, or thelike. In other examples, video decoder 300 may include more, fewer, ordifferent functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. CPB memory 320and DPB 314 may be provided by the same memory device or separate memorydevices. In various examples, CPB memory 320 may be on-chip with othercomponents of video decoder 300, or off-chip relative to thosecomponents.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, the one ormore of the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, the one or more units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the coefficient block. As discussed herein,transform processing unit 206 may selectively apply different transformsto different coefficient blocks (i.e., blocks of transformcoefficients).

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Asdiscussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures fromDPB for subsequent presentation on a display device, such as displaydevice 118 of FIG. 1.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured toderive, for a current coefficient block of a video block, a transformtype from a plurality of transform types. The video coder may transform,using the selected transform type, the current coefficient block toobtain a block of reconstructed residual data for the video block; andreconstruct, based on the reconstructed residual data for the videoblock, the video block.

FIG. 5 is a block diagram illustrating a system for hybrid videoencoding with adaptive transform selection. Video encoder 200′ of FIG. 5may be considered to illustrate a video encoding system similar to videoencoder 200 of FIGS. 1 and 3. For example, block prediction 202′, blocktransform 206′, quantization 208′, Inverse quantization 210′, Inversetransform 212′, Frame buffer 218′, and Entropy coding 220′ of videoencoder 200′ may be considered to perform operations similar to modeselection unit 202, transform processing unit 206, quantization unit208, inverse quantization unit 210, inverse transform processing unit212, decoded picture buffer 218, and entropy encoding unit 220 of videoencoder 200 of FIG. 3. As shown in FIG. 5, video encoder 200′ mayinclude transform bank 207′, which may be configured to operate inconjunction with Block transform 206′ to transform residual data. Forinstance, transform bank 207′ and block transform 206′ may collectivelyselect and perform various transforms (e.g., various DCT or DST) foreach block of prediction residuals. As discussed above, in someexamples, transform bank 207′ and block transform 206′ may signal thechoice of transform a side information. For instance, block transform206′ may cause entropy coding 220′ to encode a syntax element explicitlyindicating the transform used (i.e., t).

In some examples, Transform bank 207′ and Block transform 206′ maycompute the block transforms in a separable manner. For instance, toreduce computation complexity, transform bank 207′ and block transform206′ may transform the horizontal and vertical lines independently asshown in FIG. 6. In other words, samples along the horizontal andvertical arrows in FIG. 6 may be transformed independently.

In video coding standards prior to HEVC, only a fixed separabletransform is used where DCT-2 is used both vertically and horizontally.In HEVC, in addition to DCT-2, DST-7 is also employed for 4×4 blocks asa fixed separable transform. US-2016-0219290-A1 and US-2018-0020218-A1describe adaptive extensions of those fixed transforms, and an exampleof AMT in US-2016-0219290-A1 has been adopted in the Joint ExperimentalModel (JEM) of the Joint Video Experts Team (JVET), Joint Video ExpertsTeam (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, JEMSoftware, https://jvet.hhi.fraunhofer.de/svn/svnHMJEMSoftware/tags/HM-16.6-JEM-7.0.

In accordance with one or more techniques of this disclosure, a videocoder (e.g., a video encoder and/or a video decoder) may performimplicit transform selection. For instance, the video coder may applyone or more sets of rules to implicitly select a transform fortransforming residual data for a block. In this way, the video coder mayimprove coding efficiency. In particular, the techniques of thisdisclosure enable the video decoder to obtain the benefits of usingadaptive transform selection without the overhead of actually signalingthe transform selected.

In VVC Draft 4, there are two implicit transform derivations that arerelatively complicated and do not provide good coding performance. Thisdisclosure proposed simpler alternative derivations that may providesimilar or even better compression/coding performance/efficiency

The related techniques in VVC Draft 4 and reference software VTM-4.0)are discussed below.

In VVC Draft 4/VTM-4.0, multiple transform selection (MTS) uses ahigh-level flag to determine whether the transform is (i) explicitlysignaled to select among multiple candidates, or (ii) implicitly derivedbased on block shape. In the latter case, combinations of DST-7 andDCT-2 as horizontal or vertical transforms up to size 16. Specifically,following block-shape dependent conditions define the implicit MTS inVTM-4.0:

-   -   If the width and height of a block are equal and both are less        than or equal to 16, DST-7 is used in both horizontal and        vertical directions.    -   If the width of a block is smaller than its height and it is        less than or equal to 16, DST-7 is in horizontal and DCT-2 is        used in vertical direction.    -   If the height of a block is smaller than its width and it is        less than or equal to 16, DST-7 is in vertical and DCT-2 is used        in horizontal direction.    -   Otherwise, DCT-2 is used in both directions.

In VVC Draft 4/VTM-4.0, when intra-subblock partitioning (ISP) is usedto code luma blocks, a mode-dependent transform selection is made, wherehorizontal and vertical transforms (trTypeHor and trTypeVer) are derivedbased on the following table in VVC Draft 4.

TABLE Specification of trTypeHor and trTypeVer depending onpredModeIntra predModeIntra trTypeHor trTypeVer INTRA_ PLANAR, (nTbW >=4 && ( nTbH >= 4 && INTRA_ANGULAR31, nTbW <= 16) ? 1:0 nTbH <=16) ? 1:0INTRA_ANGULAR32, INTRA_ANGULAR34, INTRA_ANGULAR36, INTRA_ANGULAR37INTRA_ANGULAR33, 0 0 INTRA_ANGULAR35 INTRA_ANGULAR2, ( nTbW >= 4 && 0INTRA_ANGULAR4, . . ., nTbW <=16) ? 1:0 INTRA_ANGULAR28,INTRA_ANGULAR30, INTRA_ANGULAR39, INTRA_ANGULAR41, . . .,INTRA_ANGULAR63, INTRA_ANGULAR65 INTRA_ANGULAR3, 0 ( nTbH >= 4 &&INTRA_ANGULAR5, . . ., nTbH <= 16) ? 1:0 INTRA_ANGULAR27,INTRA_ANGULAR29, INTRA_ANGULAR38, INTRA_ANGULAR40, . . .,INTRA_ANGULAR64, INTRA_ANGULAR66

As discussed above and in accordance with one or more techniques of thisdisclosure, a video coder may apply one or more rule sets to implicitlyderive transform selection based on available side information.

As a first example, a video coder may determine that a codingunit/transform unit (CU/TU) is coded using DST-7 only under certainconditions. For instance, if the maximum 1-D transform size allowed is Nin a codec, the video coder may determine that DST-7 may be used for allpossible sizes. For example, for a given N×M block (as shown in FIG. 7whose N rows have M samples each, and its M columns have N samples), thevideo coder may determine that N-point DST-7 may be used vertically andM-point DST-7 may be used horizontally.

As a second example, for a selected set of dimensions, the video codermay determine that different combinations of DST-7 and DCT-2 can beused. For instance, the video coder may determine that DST-7 can beapplied for any row or column with less than or equal to K samples,while DCT-2 can be used to transform any row or column with number ofsamples greater than K. For example, in the example of FIG. 7, if N issmaller than K and M is larger than K, the video coder may determine touse N-point DST-7 vertically and M-point DCT-2 horizontally. Also in theexample of FIG. 7, if both N and M are smaller than K, the video codermay determine to use DST-7 both horizontally and vertically.

As a third example, if a CU/TU is partitioned, the video decoder maydetermine that all partitions can use the same implicit transformselection scheme. In some examples, the video coder may use DST-7 forall partitioned sub-blocks (sub TUs or sub CUs). In some examples, thevideo coder may use combinations of DST-7 and DCT-2 depending on theblock dimensions after partitioning. In some examples, for coding blocksthat use the intra-subblock partitioning (ISP) in VVC (VTM-4.0), thevideo coder may use combinations of DST-7 and DCT-2 depending on thedimensions of the block as discussed above in the second example. Forexample, for any row or column with less than or equal to 16 samples,the video coder may DST-7. Otherwise, the video coder may use DCT-2 totransform any row or column with number of samples greater than 16. Insome examples, as ISP can have rows/columns with two samples, the videocoder may use 2-point DST-7. In previous standards, 2-point DST-7 hasnot been used. As such, the video coder may use modified entries of the2-point DST-7 matrix as follows:

{48, 77}{77, −48}

As a fourth example, the video coder may derive the transform based onintra prediction modes (modes are illustrated in FIG. 8). For intraplanar and DC modes, the video coder may use DST-7 in both horizontaland vertical directions. For intra diagonal angular mode (mode index 34in FIG. 8), the video coder may use DST-7 in both horizontal andvertical directions. For angular modes, indexed from 2 to 66, the videocoder may apply different combinations of DSTs/DCTs to certain range ofmodes such as predefined intervals of mode indices between mode indices[2, 3, . . . , 65, 66].

1) The range of intervals consisting of all angular modes [2, 3, . . . ,66] can be defined as follows for a given integer T between 2 and 30:

-   -   a. R₁=[2, . . . , (33−T)]    -   b. R₂=[(34−T), . . . , (34+T)]    -   c. R₃=[(35+T), . . . , 66]

2) DST-7 can be applied both horizontally and vertically for the angularmodes in range R₂.

3) DST-7 can be applied horizontally and DCT-2 vertically for theangular modes in range R₁.

4) DCT-2 can be applied horizontally and DST-7 vertically for theangular modes in range R₃.

As a fifth example, other than DST-7 and DCT-2, the video coder mayapply combinations of different types of DCTs/DSTs (e.g., DST-4 andDCT-8) and 1-D identity transform.

As a sixth example, the video coder may apply one or more combinationsof the above examples for intra predicted CU/TUs only.

As a seventh example the video coder may apply one or more combinationsof the above examples for inter predicted CU/TUs only.

As an eighth example the video coder may apply one or more combinationsof the above examples used for both intra and inter predicted CU/TUs.

As a ninth example the video coder may apply one or more combinations ofthe above examples used for luma or chroma channels or both luma andchroma channels.

FIG. 9 is a flowchart illustrating an example method for encoding acurrent block. The current block may comprise a current CU. Althoughdescribed with respect to video encoder 200 (FIGS. 1 and 3), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 9. For instance, video encoder 200′ of FIG. 5may perform a method similar to that of FIG. 9.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate a residual blockfor the current block (352). To calculate the residual block, videoencoder 200 may calculate a difference between the original, uncodedblock and the prediction block for the current block. Video encoder 200may then transform and quantize coefficients of the residual block(354). As discussed above, video encoder 200 may implicitly derive atransform type to use when transforming the coefficients of the residualblock. For instance, video encoder 200 may derive the transform typeusing the technique discussed below with reference to FIG. 11.

Next, video encoder 200 may scan the quantized transform coefficients ofthe residual block (356). During the scan, or following the scan, videoencoder 200 may entropy encode the coefficients (358). For example,video encoder 200 may encode the coefficients using CAVLC or CABAC.Video encoder 200 may then output the entropy coded data of the block(360).

FIG. 10 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may comprise a currentCU. Although described with respect to video decoder 300 (FIGS. 1 and4), it should be understood that other devices may be configured toperform a method similar to that of FIG. 10.

Video decoder 300 may receive entropy coded data for the current block,such as entropy coded prediction information and entropy coded data forcoefficients of a residual block corresponding to the current block(370). Video decoder 300 may entropy decode the entropy coded data todetermine prediction information for the current block and to reproducecoefficients of the residual block (372). Video decoder 300 may predictthe current block (374), e.g., using an intra- or inter-prediction modeas indicated by the prediction information for the current block, tocalculate a prediction block for the current block. Video decoder 300may then inverse scan the reproduced coefficients (376), to create ablock of quantized transform coefficients. Video decoder 300 may theninverse quantize and inverse transform the coefficients to produce aresidual block (378). As discussed above, video decoder 300 mayimplicitly derive a transform type to use when transforming thecoefficients of the residual block. For instance, video decoder 300 mayderive the transform type using the technique discussed below withreference to FIG. 11. Video decoder 300 may ultimately decode thecurrent block by combining the prediction block and the residual block(380).

FIG. 11 is a flowchart illustrating an example method for inferring atransform type for a transform block of a video block, in accordancewith one or more techniques of this disclosure. The techniques of FIG.11 may be performed by a video coder (e.g., video encoder 200 and/orvideo decoder 300).

A video coder may obtain a current transform block of a current videoblock (1102). The transform block may be a matrix of transformcoefficients that is constructed based on one or more syntax elementsdecoded from a video bitstream (e.g., the syntax elements included inthe residual coding syntax table of VVC Draft 4. The current video blockmay be a coding unit (CU).

The video coder may infer a transform type from a plurality of transformtypes for the current transform block. The plurality of transform typesmay include one or more discrete cosine transforms (DCTs) and one ormore discrete sine transforms (DSTs).

As discussed above, the video coder may infer the transform type basedon one or more factors, such as whether the current video block ispartitioned using ISP and/or a size of the transform block. As shown inFIG. 11, the video coder may determine that the current video block ispartitioned using ISP (1104). The video coder may determine that thecurrent video block is partitioned using ISP based on the values of oneor more syntax elements (e.g., sps_isp_enabled_flag,intra_subpartitions_mode_flag, and/or intra_subpartitions_split_flag).For instance, based on the intra_subpartitions_split_flag syntaxelement, the video coder may determine whether the current video blockis not partitioned (e.g., not split), is partitioned horizontally, or ispartitioned vertically.

Responsive to determining that the current video block is partitionedusing ISP (1104), the video coder may determine a size of the currenttransform block (1106). For instance, the video coder may determine awidth and/or a height of the transform block. In some examples, thevideo coder may separately determine transform block size for eachsub-partition. In other examples, the video coder may determinetransform block size for a single partition and utilize the determinedsize for each partition of the coding unit.

The video coder may determine whether the size of the current transformblock satisfies a size threshold. For instance, as shown in FIG. 11, thevideo coder may determine whether the size of the current transformblock is greater than a lower bound and less than an upper bound (i.e.,whether both (size>lower bound) and (size<upper bound) are true) (1108).As discussed above, in some examples, the lower bound may be 4 samplesand the upper bound may be 16 samples).

Responsive to determining that the size of the current transform blocksatisfies the size threshold and that the current video block ispartitioned using ISP, the video coder may select a particular DST ofthe one or more DSTs as the selected transform type. For instance, asshown in FIG. 11, responsive to determining that the size of the currenttransform block satisfies the size threshold and that the current videoblock is partitioned using ISP, the video coder may select DST-7 as theinferred transform type for the current transform block (“Yes” branch of1108, 1110). Alternatively, responsive to determining that the size ofthe current transform block does not satisfy the size threshold and thatthe current video block is partitioned using ISP, the video coder mayselect DCT-2 as the inferred transform type for the current transformblock (“No” branch of 1108, 1112).

The video coder may transform, using the selected transform type, thecurrent transform block to obtain a block of reconstructed residual datafor the video block (1114). For instance, where the selected transformtype is DST-7, the video coder (e.g., inverse transform processing unit212/212′ of video encoder 200/200′ and/or inverse transform processingunit 308 of video decoder 300) may transform the coefficients of thetransform block into the reconstructed residual data by applying aninverse DST-7 transform.

The video coder may reconstruct, based on the reconstructed residualdata for the video block, the video block (1116). For instance, thevideo encoder may add the residual data to a block of intra predictedsamples for the current block. Where the video block is partitionedusing ISP, the video encoder may add a respective block of reconstructedresidual data to a respective block of intra predicted samples for eachrespective sub-partition of the current video block.

The following numbered examples may illustrate one or more aspects ofthe disclosure:

Example 1. A method of coding video data, the method comprising:deriving, for a current coefficient block of a video block, a transformtype from a plurality of transform types; transforming, using theselected transform type, the current coefficient block to obtain a blockof reconstructed residual data for the video block; and reconstructing,based on the reconstructed residual data for the video block, the videoblock.

Example 2. The method of example 1, wherein the plurality of transformtypes includes one or more discrete cosine transforms (DCTs) and/or oneor more discrete sine transforms (DSTs).

Example 3. The method of example 2, wherein the one or more DCTs includeone or more of a DCT-1, a DCT-2, a DCT-3, a DCT-4, a DCT-5, a DCT-6, aDCT-7, and a DCT-8.

Example 4. The method of any of examples 2 and 3, wherein the one ormore DSTs include one or more of a DST-1, a DST-2, a DST-3, a DST-4, aDST-5, a DST-6, a DST-7, and a DST-8.

Example 5. The method of any of examples 1-4, wherein deriving thetransform type comprises deriving the transform type based on a size ofthe current coefficient block.

Example 6. The method of example 5, wherein deriving the transform typebased on the size of the current coefficient block comprises selectingthe DST-7 transform type where a maximum 1-D transform size allowed isN.

Example 7. The method of example 6, wherein the current coefficientblock has dimensions of N×M, and wherein selecting the DST-7 transformtype comprises selecting an N-point DST-7 transform for vertical use andselecting an M-point DST-7 transform for horizontal use.

Example 8. The method of any combination of examples 1-7, whereinderiving the transform type comprises selecting different combinationsof the DST-7 transform and the DCT-2 transform.

Example 9. The method of example 8, wherein selecting differentcombinations of the DST-7 transform and the DCT-2 transform comprises:selecting the DST-7 transform for any row or column with less than orequal to K samples; and selecting the DCT-2 transform for any row orcolumn with greater than K samples.

Example 10. The method of any combination of examples 1-9, furthercomprising: responsive to determining that the video block ispartitioned into a plurality of partitions, selecting respectivetransform types for coefficient blocks of each of the plurality ofpartitions using a common rule set.

Example 11. The method of example 10, wherein selecting respectivetransform types for each of the plurality of partitions comprisesselecting the DST-7 for coefficient blocks of all of the plurality ofpartitions.

Example 12. The method of example 10, wherein selecting respectivetransform types for each of the plurality of partitions comprisesselecting different combinations of the DST-7 transform and the DCT-2transform based on dimensions of the partitions.

Example 13. The method of example 12, wherein selecting differentcombinations of the DST-7 transform and the DCT-2 transform based ondimensions of the partitions comprises: selecting the DST-7 transformfor any row or column with less than or equal to a threshold number ofsamples; and selecting the DCT-2 transform for any row or column withgreater than the threshold number of samples.

Example 14. The method of example 13, wherein the threshold is 16.

Example 15. The method of any combination of examples 10-14, whereinpartitioning the video block into the plurality of partitions comprisespartitioning the video block using intra-subblock partitioning (ISP).

Example 16. The method of example 15, wherein transforming using theDST-7 transform comprises transforming the current coefficient blockusing the following 2-point DST-7 matrix:

{48, 77} {77, −48}.

Example 17. The method of any combination of examples 1-16, furthercomprising: determining an intra prediction mode used to predict thevideo block, wherein deriving the transform type for the currentcoefficient block of the video block comprises deriving the transformtype for the current coefficient block of the video block based on theintra prediction mode.

Example 18. The method of example 17, wherein deriving the transformtype for the current coefficient block of the video block based on theintra prediction mode comprises: responsive to determining that theintra prediction mode is a planar or a DC mode, selecting the DST-7transform for the current coefficient block in both horizontal andvertical directions.

Example 19. The method of any of examples 17 or 18, wherein deriving thetransform type for the current coefficient block of the video blockbased on the intra prediction mode comprises: responsive to determiningthat the intra prediction mode is a diagonal angular mode, selecting theDST-7 transform for the current coefficient block in both horizontal andvertical directions.

Example 20. The method of example 19, wherein the diagonal angular modeis mode index 34.

Example 21. The method of any of examples 17-20, wherein deriving thetransform type for the current coefficient block of the video blockbased on the intra prediction mode comprises: responsive to determiningthat the intra prediction mode is an angular mode, selecting thetransform type for the current coefficient block based on a mode indexof the intra prediction mode.

Example 22. The method of example 21, wherein selecting the transformtype for the current coefficient block based on the mode index of theintra prediction mode comprises: identifying a range of a plurality ofranges that includes the mode index of the intra prediction mode; andselecting the transform type for the current coefficient block based onthe identified range.

Example 23. The method of example 22, wherein identifying the rangecomprises: identifying a first range in response to determining that themode index is between a first threshold and a second threshold;identifying a second range in response to determining that the modeindex is between the second threshold and a third threshold; andidentifying a third range in response to determining that the mode indexis between the third threshold and a fourth threshold.

Example 24. The method of example 23, wherein: identifying the firstrange in response to determining that the mode index is between thefirst threshold and the second threshold comprises identifying the firstrange in response to determining that the mode index is within [2, . . ., (33−T)]; identifying the second range in response to determining thatthe mode index is between the second threshold and the third thresholdcomprises identifying the second range in response to determining thatthe mode index is within [(34−T), . . . , (34+T)]; identifying a thirdrange in response to determining that the mode index is between thethird threshold and the fourth threshold comprises identifying the thirdrange in response to determining that the mode index is within [(35+T),. . . , 66]; and T is an integer between 2 and 30.

Example 25. The method of example 23 or example 24, wherein selectingthe transform type for the current coefficient block based on theidentified range comprises: selecting the DST-7 for horizontal use andthe DCT-2 for vertical use in response to identifying the first range;selecting the DST-7 for horizontal and vertical use in response toidentifying the second range; and selecting the DCT-2 for horizontal useand the DST-7 for vertical use in response to identifying the thirdrange.

Example 26. The method of any of examples 1-25, wherein coding comprisesdecoding.

Example 27. The method of any of examples 1-26, wherein coding comprisesencoding.

Example 28. A device for coding video data, the device comprising one ormore means Example for performing the method of any of examples 1-27.

Example 29. The device of example 28, wherein the one or more meanscomprise one or more processors implemented in circuitry.

Example 30. The device of any of examples 28 and 29, further comprisinga memory to store the video data.

Example 31. The device of any of examples 28-30, further comprising adisplay configured to display decoded video data.

Example 32. The device of any of examples 28-31, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Example 33. The device of any of examples 28-32, wherein the devicecomprises a video decoder.

Example 34. The device of any of examples 28-33, wherein the devicecomprises a video encoder.

Example 35. A computer-readable storage medium having stored thereoninstructions that, when executed, cause one or more processors toperform the method of any of examples 1-25.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processing circuity,”as used herein may refer to any of the foregoing structures or any otherstructure suitable for implementation of the techniques describedherein. In addition, in some aspects, the functionality described hereinmay be provided within dedicated hardware and/or software modulesconfigured for encoding and decoding, or incorporated in a combinedcodec. Also, the techniques could be fully implemented in one or morecircuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of decoding video data, the methodcomprising: inferring, for a current transform block of a current videoblock, a transform type from a plurality of transform types thatincludes one or more discrete cosine transforms (DCTs) and one or morediscrete sine transforms (DSTs), wherein inferring the transform typecomprises: determining a size of the current transform block;determining whether the current video block is partitioned usingintra-subblock partitioning (ISP); and responsive to determining thatthe size of the current transform block satisfies a size threshold andthat the current video block is partitioned using ISP, selecting aparticular DST of the one or more DSTs as the selected transform type;transforming, using the selected transform type, the current transformblock to obtain a block of reconstructed residual data for the videoblock; and reconstructing, based on the reconstructed residual data forthe video block, the video block.
 2. The method of claim 1, wherein theone or more DCTs include one or more of a DCT-1, a DCT-2, a DCT-3, aDCT-4, a DCT-5, a DCT-6, a DCT-7, and a DCT-8.
 3. The method of claim 2,wherein the one or more DSTs include one or more of a DST-1, a DST-2, aDST-3, a DST-4, a DST-5, a DST-6, a DST-7, and a DST-8.
 4. The method ofclaim 3, wherein determining that the size of the current transformblock satisfies the size threshold comprises determining that the sizeof the current transform block is greater than a lower bound and lessthan an upper bound.
 5. The method of claim 4, wherein selecting theparticular DST comprises selecting the DST-7 responsive to determiningthat the size of the current transform block satisfies the sizethreshold and that the current video block is partitioned using ISP. 6.The method of claim 5, further comprising: responsive to determiningthat the size of the current transform block does not satisfy the sizethreshold and that the current video block is partitioned using ISP,selecting a particular DCT of the one or more DCTs as the selectedtransform type.
 7. The method of claim 6, wherein selecting theparticular DCT comprises selecting the DCT-2 responsive to determiningthat the size of the current transform block does not satisfy the sizethreshold and that the current video block is partitioned using ISP. 8.The method of claim 5, wherein selecting the transform type comprisesselecting the transform type regardless of an intra prediction mode usedto predict the current video block.
 9. The method of claim 8, whereindetermining the size of the current transform block comprises:determining a width of the current transform block; and determining aheight of the current transform block.
 10. The method of claim 9,wherein selecting the transform type comprises selecting a transformtype for horizontal use and selecting a transform type for vertical use,the method further comprising: selecting the DST-7 as the selectedtransform type for horizontal use responsive to determining that thewidth of the current transform block satisfies a width size thresholdand that the current video block is partitioned using ISP; and selectingthe DST-7 as the selected transform type for vertical use responsive todetermining that the height of the current transform block satisfies aheight size threshold and that the current video block is partitionedusing ISP.
 11. The method of claim 10, wherein the width thresholdequals the height threshold.
 12. The method of claim 11, wherein thewidth threshold and the height threshold are both 16 samples.
 13. Themethod of claim 12, wherein inferring the transform type for the currenttransform block comprises inferring the transform type for the currenttransform block responsive to determining that multiple transformselection (MTS) is enabled for the current video block.
 14. The methodof claim 8, wherein whether the current video block is partitioned usingISP comprises: determining, based on values of one or more syntaxelements decoded from a video bitstream, whether the current video blockis partitioned using ISP.
 15. A device for coding video data, the devicecomprising: a memory configured to store video blocks; and one or moreprocessors implemented in circuitry and configured to: infer, for acurrent transform block of a current video block, a transform type froma plurality of transform types that includes one or more discrete cosinetransforms (DCTs) and one or more discrete sine transforms (DSTs),wherein, to infer the transform type, the one or more processors areconfigured to: determine a size of the current transform block;determine whether the current video block is partitioned usingintra-subblock partitioning (ISP); and select, responsive to determiningthat the size of the current transform block satisfies a size thresholdand that the current video block is partitioned using ISP, a particularDST of the one or more DSTs as the selected transform type; transform,using the selected transform type, the current transform block to obtaina block of reconstructed residual data for the video block; andreconstruct, based on the reconstructed residual data for the videoblock, the video block.
 16. The device of claim 15, wherein the one ormore DCTs include one or more of a DCT-1, a DCT-2, a DCT-3, a DCT-4, aDCT-5, a DCT-6, a DCT-7, and a DCT-8.
 17. The device of claim 16,wherein the one or more DSTs include one or more of a DST-1, a DST-2, aDST-3, a DST-4, a DST-5, a DST-6, a DST-7, and a DST-8.
 18. The deviceof claim 17, wherein, to determine that the size of the currenttransform block satisfies the size threshold, the one or more processorsare configured to determine that the size of the current transform blockis greater than a lower bound and less than an upper bound.
 19. Thedevice of claim 18, wherein, to select the particular DST, the one ormore processors are configured to select the DST-7 responsive todetermining that the size of the current transform block satisfies thesize threshold and that the current video block is partitioned usingISP.
 20. The device of claim 19, wherein the one or more processors arefurther configured to: select, responsive to determining that the sizeof the current transform block does not satisfy the size threshold andthat the current video block is partitioned using ISP, a particular DCTof the one or more DCTs as the selected transform type.
 21. The deviceof claim 20, wherein, to select the particular DCT, the one or moreprocessors are configured to select the DCT-2 responsive to determiningthat the size of the current transform block does not satisfy the sizethreshold and that the current video block is partitioned using ISP. 22.The device of claim 19 wherein, to select the transform type, the one ormore processors are configured to select the transform type regardlessof an intra prediction mode used to predict the current video block. 23.The device of claim 22, wherein, to determine the size of the currenttransform block, the one or more processors are configured to: determinea width of the current transform block; and determine a height of thecurrent transform block.
 24. The device of claim 23, wherein, to selectthe transform type, the one or more processors are configured to selecta transform type for horizontal use and selecting a transform type forvertical use, and wherein the one or more processors are furtherconfigured to: select the DST-7 as the selected transform type forhorizontal use responsive to determining that a width of the currenttransform block satisfies a width size threshold and that the currentvideo block is partitioned using ISP; and select the DST-7 as theselected transform type for vertical use responsive to determining thata height of the current transform block satisfies a height sizethreshold and that the current video block is partitioned using ISP. 25.The device of claim 24, wherein the width threshold equals the heightthreshold.
 26. The device of claim 25, wherein the width threshold andthe height threshold are both 16 samples.
 27. The device of claim 26,wherein, to infer the transform type for the current transform block,the one or more processors are configured to infer the transform typefor the current transform block responsive to determining that multipletransform selection (MTS) is enabled for the current video block.
 28. Acomputer-readable storage medium storing instructions that, whenexecuted, cause one or more processors of a video coding device to:infer, for a current transform block of a current video block, atransform type from a plurality of transform types that includes one ormore discrete cosine transforms (DCTs) and one or more discrete sinetransforms (DSTs), wherein the instructions that cause the one or moreprocessors to infer the transform type comprise instructions that causethe one or more processors to: determine a size of the current transformblock; determine whether the current video block is partitioned usingintra-subblock partitioning (ISP); and select, responsive to determiningthat the size of the current transform block satisfies a size thresholdand that the current video block is partitioned using ISP, a particularDST of the one or more DSTs as the selected transform type; transform,using the selected transform type, the current transform block to obtaina block of reconstructed residual data for the video block; andreconstruct, based on the reconstructed residual data for the videoblock, the video block.
 29. The computer-readable storage medium ofclaim 28, wherein the one or more DSTs comprise at least a DST-7,wherein the instructions that cause the one or more processors to selectthe particular DST comprise instructions that cause the one or moreprocessors to select the DST-7 regardless of an intra prediction modeused to predict the current video block responsive to determining thatthe size of the current transform satisfies the size threshold and thatthe current video block is partitioned using ISP.